This is fairly long, but is a much better article about setting up/fitting a BMX than I could ever write. It's primarily oriented towards BMX racing, but it's also exactly the sort of no-bull**** article you need to read for setting up a bike for just cruising around on.

http://www.bmxbasics.org/new/bmx0695.html
http://www.bmxbasics.org/new/bmx0895.html